This invention is directed to threshold switching devices which exhibit negative differential resistance (NDR), and to certain ceramic threshold switching devices which are useful in synthetic neural networks.
The brain distributes its processing tasks among billions of relatively unintelligent nerve cells called neurons. Neural networks are attempts to simulate brainlike functions on conventional computers. Thus, a "synthetic neural network" is the structure of a machine modeled after the brain but not the actions that it performs. Such networks seek to mimic the properties of neurons with electronic components.
A synapse is the contact point between two neurons. A simple network includes a number of such contact points or connections between an input and an output. Values or "weights" are assigned to these connections. Additional layers can be added to the network between the input and the output in the form of hidden units.
It has been found that a device including a silicon dioxide thin film and electrodes, is capable of functioning as an electrical component in the construction of such a synthetic neural network. The device is utilized in the network as a variable resistive element, and in this capacity, the device operates as a synapse with weighting factors between adjacent layers of neuronal processing nodes.
In this application of the device, the resistance of each two-terminal device can be adjusted by applying a voltage pulse of sufficient magnitude to the electrodes of the device, in accordance with a back-propagation algorithm or other representations of the internal learning process which are used to control the learning function of a neural network.
Devices which exhibit threshold switching, and metal oxide devices which exhibit threshold switching with negative differential resistance (NDR) are known in the art. The switching and negative differential resistance (NDR) characteristics of silicon oxide films has also been described in the literature.
The use of thin film silica coatings derived from hydrogen silsesquioxane resin to provide protection and electrical insulation is not new, but the use of those coatings to form switching devices which are useful as synapses is believed to be novel.
Coatings and switching devices formed by depositing a thin hydrogen silsesquioxane derived silicon dioxide film between at least two electrodes and applying a voltage above a threshold voltage across the electrodes, are described in detail in a prior patent application U.S. Ser. No. 07/694,721 filed May 2, 1991, and entitled "Threshold Switching Devices", now U.S. Pat. No. 5,312,684. The present invention involves these and similar devices and the utility of such variable resistors as synapses in neural networks.